A signal processing device which outputs a discrete signal composed of a string of the sampling values and parameters m signal. The signal processing device includes a sampling circuit which samples an input signal and outputs a discrete signal, multiple function generators which generate multiple sampling functions with parameters m different from each other, plural inner product operating units for each of parameters m that take an inner product between the input signal and each of plural sampling functions and output an inner product operating value, and a judging unit which determines parameter m providing a minimum error out of multiple errors composed of differences between the sampling value and inner product operating values output from the multiple inner product operating units and outputs the parameters m signal.